Macromolecules 2004, 37, 5485-5489
5485
Articles Controlled Synthesis of High Molecular Weight Telechelic Polybutadienes by Ring-Opening Metathesis Polymerization Shengxiang Ji,† Thomas R. Hoye,*,† and Christopher W. Macosko‡ Department of Chemistry and Department of Chemical Engineering and Materials Science, University of Minnesota, 207 Pleasant St. S.E., Minneapolis, Minnesota 55455 Received April 8, 2004; Revised Manuscript Received May 24, 2004
ABSTRACT: High molecular weight telechelic [dicyano- and dichloro-] polybutadienes (TPBs) have been synthesized via ring-opening metathesis (ROM) polymerization of 1,5-cyclooctadiene (COD) in the presence of the difunctional chain transfer agents (CTAs), 1,4-dicyano- or 1,4-dichloro-2-butene. To achieve high molecular weights, it was critical that the COD monomer contained very low levels of the contaminating, isomeric 4-vinylcyclohexene (VCH), since the latter serves as a competing CTA. An NMR method for determining the level of VCH was developed as well as “reactive purification” strategy in which the VCH was selectively depleted by its (faster) hydroboration. A “two-stage” protocol for preparation of the TPBs was developed wherein a small portion of the eventual charge of COD and the entire quantity of the CTA were first copolymerized to produce telechelic oligomeric butadiene (TOB) of low viscosity. After all of the CTA had been consumed, the remaining COD was charged. This protocol allowed for better MW control and for higher functionality in the resulting TPBs. Finally, diamino-terminated TPBs were prepared by lithium aluminum hydride reduction of the precursor dicyano-TPBs.
Introduction R,ω-Difunctional (telechelic) polymers have attracted much interest due to their unique properties and synthetic challenges.1 These polymers bear functional groups at both termini and have potential applications as, e.g., pressure-sensitive adhesives,2 compatibilizers for polymer blends,3 binders for composite materials (e.g., rocket propellant binders),4 building blocks for segmented block copolymers,5 and cross-linking agents for polymer networks. Styrene-butadiene-styrene (SBS) block copolymers containing central polybutadiene (PB) blocks of high molecular weight are particularly attractive in their potential for use in pressure-sensitive adhesives. However, premade SBS block copolymers are difficult to process due to their high order-disorder transition temperature and high viscosity. An attractive strategy for approaching this problem would be the use of appropriate reactive coupling reactions in the melt between high molecular weight functional telechelic PBs (TPBs) to generate the SBS block copolymers in situ.3,6 Telechelic polybutadienes with different functional groups, such as dihydroxyl (e.g., Krasol LBH and Poly Bd),7 diamino,8 dichloro,9 diepoxy,10 diacrylate,10 and dicarboxylic acid,8c,11 have been synthesized by radical polymerization, anionic polymerization, or ring-opening metathesis (ROM) polymerization. Among these, ROM polymerization is an established and reliable strategy for the synthesis of TPBs having high functionality, especially for linear 1,4-PBs. However, most of the †
Department of Chemistry. Department of Chemical Engineering and Materials Science. * Corresponding author: e-mail
[email protected].
‡
previously reported telechelic polybutadienes prepared by ROM polymerization are of relatively low molecular weight (most 500 ppm) of the VCH, so yet more
Macromolecules, Vol. 37, No. 15, 2004
Telechelic Polybutadienes 5487
Table 2. Synthesis of Telechelic Polybutadienes 3 Using Purified COD (1) and 1,4-Dicyano- (2b) or 1,4-Dichloro-2-butene (2a) by Two-Stage ROM Polymerization
a
CTA
monomer:CTA ratio [COD (1)]/[CTA (2)]
initiator (mol %)a
COD conv (%)
CTA conv (%)
calcd Mnb (K)
obsd Mnc (K)
PDI
2b 2b 2b 2b 2b 2a
200:1 400:1 600:1 800:1 1000:1 2000:1
2 3 3 4 4 5
100 100 92.0 92.7 88.5 87.8
100 100 100 100 100 100
21.6 43.2 59.6 80.0 95.6 190
20.3 39.8 54.0 74.5 90.3 196
1.97 1.57 1.59 1.80 1.79 1.37
Based on CTA. b Theoretical Mn × COD % conversion. c Measured by GPC using a PS standard and THF elution (universal calibration).
Scheme 2
Figure 1. 1H NMR spectra of (A) commercial COD, containing ∼1000 ppm VCH, (B) commercial COD following treatment with increasing amounts of H3B‚THF, and (C) purified COD (
